Something that struck me immediately when watching Luca the first time was the different ways that emotions were shown on the underdogs' faces in this scene:
It's only split seconds within the scene but the slight differences on their faces speak volumes.
Giulia looks at her father with a pleading look. Her face says "Please, dad." She's the one who knows him best and fears for her friends, wondering how he will act. She's torn between the trust she has for him and the fear that his passion for hunting sea monsters might take over. She's the one who's known him the longest, and yet she has no idea what to expect.
Luca looks at Massimo with that defiant look. His face says nothing less than "Try me, bitch." Out of the three of them, he has the least of an emotional connection with Massimo. To him it's clear: As soon as the man takes one wrong step, Luca will jump to protect his friend and himself. He is not ashamed of his sea monster form anymore and this is so special because it contrasts Luca's tendency to avoid conflict and exposure throughout the entire story. He ran away to leave his sea monster home behind, he betrayed Alberto to keep his identity a secret and his greatest wish was to live as a full on human and go to school. Now, Luca is decisive, protective instead of protected. He's out in the open, he is not ashamed anymore and he won't fuck around.
Then there's Alberto. Alberto, the boy who insists he always has everything under control, who never hesitates to jump and take a risk, is barely able to look Massimo in the eyes, riddled with shame. He knows how Massimo feels about sea monsters, knows that the man wouldn't hesitate to stab through their heart with a harpoon when he gets the chance. But he also knows Massimo's reassuring shoulder touches. He knows the approving looks the man has sent him whenever he's impressed. He knows the connection he's started to feel with the man, one he's felt before with a father that left him behind.
Here he takes a small fearful step back and looks to the ground, not daring to meet the man's eye. Alberto's laid bare, vulnerable and exposed in his full form for Massimo to see. Massimo, who hates sea monsters. Massimo, who will most definitely turn his back on him now too, because there is just nothing to Alberto that seems worth keeping. He was left once, he can be left again.
He looks up to the man hesitantly and just barely, bracing himself for another rejection. He doesn't even seem to have the energy to hide the sadness on his face, that's how broken it is.
He almost looks like he's apologizing, like he's saying "I'm sorry, this is what I am."